A heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) which uses differing semiconductor materials for the emitter and base (or base and collector) regions, creating a heterojunction. The HBT can handle signals of very high frequencies, up to several hundred GHz. It is commonly used in modern circuits, mostly radio-frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in cellular phones.
To increase the performance of the current generation of HBTs it is necessary to lower parasitic resistances and capacitances such as base resistance (Rb), collector base capacitance (Ccb) and base-emitter capacitance (Cbe). A higher ft/fmax and improved kirk current limit will also increase performance of the HBT.